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Calcimator

Roundabout Capacity Calculator

Calculate entry capacity, volume-to-capacity ratio, delay, and level of service for single and multi-lane roundabouts using HCM methodology.

Inputs

veh/hr
ft
ft
°

Results

Entry Capacity (per lane)

685 veh/hr

Level of Service

B

Total Entry Capacity685 veh/hr
V/C Ratio0.44
Control Delay14.3 sec/veh
95th % Queue (est.)156 ft
How to Use This Calculator
  1. Enter Entry Lanes, 1 Lane, and 2 Lanes.
  2. Set Circulating Lanes, 1 Lane, and 2 Lanes.
  3. Adjust Conflicting Circulating Flow, Entry Width as needed.
  4. Review Entry Capacity (per lane) (veh/hr) and Level of Service.
  5. Use Total Entry Capacity (veh/hr) and V/C Ratio to inform your decision.

How the result changes with Conflicting Circulating Flow

Conflicting Circulating FlowEntry Capacity (per lane)Level of Service
300837 veh/hrB
1,050395 veh/hrF
1,950161 veh/hrF
2,70076 veh/hrF

What each input means

Entry Lanes
Number of lanes at the roundabout entry approach.
Circulating Lanes
Number of lanes in the circulatory roadway.
Conflicting Circulating Flow
Volume of traffic circulating past the entry point during the peak hour.
Entry Width
Width of the entry at the yield line. Typical single-lane entry is 14-18 ft.
Inscribed Circle Diameter
Diameter of the largest circle that fits within the outer curb. Mini: 65-90 ft, single-lane: 105-150 ft.
Entry Angle
Angle between the entry and circulating roadway. Typically 20-40 degrees.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Entry Lanes = 1, Circulating Lanes = 1, Conflicting Circulating Flow = 500, Entry Width = 15 = 6 input(s) provided
  2. Calculate Entry Capacity
    Entry Capacity
    685 = 685
  3. Calculate Level of Service
    B = B
  4. Calculate Total Entry Capacity
    Total Entry Capacity
    685 = 685
  5. Calculate V/C Ratio
    V/C Ratio
    0.438 = 0.438

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